I just read Jane Yolen's answer to how she came to write Briar Rose and realized someone should do an article on books that began with a notion by Terri Windling. War for the Oaks is among them. So is the Liavek series—Terri knew of our group of writers and thought we could have fun doing a shared-world anthology like Thieves' World with more of a high fantasy take.
ETA: To be clear, War for the Oaks didn't begin as an idea by Terri. Emma was working on Falcon when she mentioned her notion for War for the Oaks to Terri. Terri told Emma that if she put Falcon aside and started War for the Oaks immediately, Terri would give her a contract. Being a wise beginning writer, Emma did just that.